Lasers and laser amplifiers can be energized by many different means and may use many different laser media. Of special interest here are laser amplifiers energized or "pumped" by laser diodes. For a review of laser diode pumped solid-state lasers see T. Y. Fan and R. L. Byer, "Diode laser-pumped solid-state lasers", I.E.E.E. Jour. of Quant. Elec., vol. 24 (1988) pp. 895-912. Here the term "solid-state lasers" includes all lasers, except semiconductor laser diodes (referred to as "laser diodes"), in which the laser gain medium is a solid-state material. Original work in laser diodes, and a laser diode's applicability to pumping of solid-state lasers, is discussed by W. Streifer et al, "Phased array diode lasers", Laser Focus/Electro-optics, June 1984, pp. 100-109, by R. J. Keys and T. M. Quist, "Injection luminescent pumping of CaF.sub.2 :U.sup.3+ with GaAs diode lasers", Appl. Phys. Lett., vol. 4 (1963) pp. 50-52, and by M. Ross, "YAG laser operation by semiconductor laser pumping", Proc. I.E.E.E., vol. 56 (1968) pp. 196-197. New high power laser diodes can be used for laser diode pumping. However, the pump light must overlap the mode of the solid-state laser to obtain efficient laser operation, and a good geometry for doing this is end-pumping. These are discussed by L. J. Rosencrantz, "GaAs diode-pumped Nd:YAG laser", Jour. Appl. Phys., vol. 43 (1973) pp. 4603-4605, by K. Kubodera and J. Noda, "Pure single mode LiNdP.sub.4 O.sub.12 solid-state laser transmitter for 1.3 .mu.m fiber-optic communications", Appl. Optics, vol. 21 (1982) pp. 3466-3469, and by D. L. Sipes, "Highly efficient neodymium:yttrium aluminum garnet laser end pumped by a semiconductor laser array", Appl. Phys. Lett., vol. 47 (1985) pp. 74-76.
Thermal lensing in the laser gain media can perturb the laser or amplifier optics and degrade the mode overlap in the media. In addition, light from higher power laser diodes is often far from diffraction limited; such light does not appear in a single transverse spatial mode. Work has been applied toward designing solid-state laser cavities and optics that transfer diode light so that the pump light and the solid-state laser mode efficiently overlap in the laser gain medium.
For many applications increased laser power is useful. One way to increase power is to make a more powerful laser oscillator. But, as with electronic oscillators, it is often easier to obtain desired characteristics from a small, well-controlled laser oscillator, and then amplify the oscillator's output to obtain higher powers. Amplifiers potentially can scale short-pulse or frequency-stable laser oscillators to high power.
Laser amplifiers can increase optical power and are as old as lasers themselves. Linear amplifiers, fiber amplifiers, and multi-pass amplifiers have been built, and are discussed in the references cited below. However, much of the work on amplifiers using diode-pump sources has been on two categories of amplifiers. The first is diode-pumped fiber amplifiers for communications applications. Fiber amplifiers have tremendous utility as low-noise high-gain amplifiers, but they are not ideally suited for use in high peak or average power applications because the fiber may damage, and it is difficult to pump single-transverse-mode fiber with high power diodes that are not themselves single transverse mode. See J. D. Minelly et al, "Laser diode-pumped neodymium-doped fiber laser with output power&gt;1 W", paper CWE6, Conference on Lasers and Electro-optics, 1992, Digest of Technical Papers, Opt. Soc. of America, Washington, D.C., for a particular approach to this problem. The second category of amplifier is single- or double-pass amplifiers where for efficiency the input must be energetic enough to saturate the amplifier. These amplifiers in the master oscillator/power amplifier configuration are technologically very important. However, these lasers are not generally optimized for large gain, but rather for high power, high energy, and high extraction efficiency. One example of an amplifier that does not fall directly into the two broad categories above is the tightly folded amplifier or resonator disclosed by T. M. Baer in U.S. Pat. Nos. 4,785,459, 4,894,839 and 4,908,832. Another example is a precessing slab amplifier discussed by D. B. Coyle, "Design of a high gain laser diode array-pumped Nd:YAG alternating precessive slab amplifier", I.E.E.E. Jour. Quant Elec., vol. 27 (1991) pp. 2327-2331.
R. P. Sandoval, in "Angular multiplexing as a technique for short-pulse amplification in a high gain xenon amplifier", Jour. Appl. Phys., vol. 49 (1978) pp. 5745-5749, notes that passing a sequence of light beams through the same volume of an amplifier but at different angles will amplify short pulses, if a significant fraction of the available energy is extracted with each pass and if amplified spontaneous emission is suppressed. W. M. Grossman et al, in "Axisymmetric angular encoder for laser fusion", I.E.E.E. Jour. Quant. Elec., vol. QE-17 (1981) pp. 1870-1878, disclose use of a multi-pass refocusing ring laser amplifier, which is only marginally stable when operating as an oscillator, using angle mutiplexing; the optical ring is purposely misaligned relative to the injected beam. W. R. Trutna and R. L. Byer, in "Multiple-pass Raman gain cell", Appl. Optics, vol. 19 (1980) pp. 301-312, describe use of a stable resonator for multi-pass amplification of Raman radiation, using angle multiplexing; the successive light beams do not pass through a single point in the gain medium.
Even with higher power laser diode sources, the efficient end-pumped configuration is commonly believed to limit the amount of energy that can be used (as stated by Baer, Welford et al, and Tidwell et al; see the citations in this patent), thereby limiting the power of the laser, since the power densities in the pump region of the gain medium become too high and the heat produced cannot be removed. One difficulty encountered with large heat deposition in oscillators is that heat flow results in thermal lensing and aberrations in the laser gain medium and can also lead to thermal birefringence and thermal fracture and loss of efficiency. Thermal lensing is inherent in high power side-pumped or end-pumped lasers. One technique to reduce thermal aberrations in resonators is to apply compensating optics as suggested by S. C. Tidwell et al, "Scaling CW diode end-pumped Nd:YAG lasers to high average powers", I.E.E.E. Jour. Quant Elec., vol. 28 (1992) pp. 997-1009.
Another approach is to design a laser cavity with an elliptical optical mode shape as suggested by R. W. Wallace et al, "Elliptical Mode Cavities for Solid-state Lasers Pumped by Laser Diodes", U.S. Pat. No. 5,103,457. Use of an elliptical mode, rather than a circular mode, aids in heat transfer and helps match the cavity mode to the shape of the pumped volume provided by some higher power laser diode sources. However, in diode-pumped amplifiers to date, avoiding the effects of thermal focusing and thermal aberrations has not been adequately addressed. The subject invention discloses a technique for efficient laser amplification that can give high gain and high efficiency with reduced sensitivity to thermal lensing in the laser amplifier media. This technique can accept end-pumped or side-pumped geometries and is not restricted to use of laser diode pumps for laser gain medium pumping.
Some side-pumped diode-pumped amplifiers and variations on end-pumped diode-pumped amplifiers have been built. In these amplifiers the ability to produce high gain is still impaired by thermal lensing. In T. M. Baer et al, "Performance of diode pumped Nd:YAG and Nd:YLF lasers in a tightly folded resonator configuration", I.E.E.E.Jour. Quant Elec., vol. 28 (1992) pp. 1131-1138, the authors state that the tightly folded design is difficult to use due to thermal lensing, when using the important laser medium, Nd:YAG. The subject invention produces greater gain with less pump power.
The geometry of the subject invention bears some resemblance to the earlier work of E. V. Khoroshilov et al, "10 kHz-Rate Amplification of 40-fs Optical Pulses at Low Pumping Energy", Springer Series in Chemical Physics, vol. 48, Ultrafast Phenomena VI, Springer Verlag, Berlin/Heidelberg, 1988, pp. 22-23 and the work of P. Georges et al, "High efficiency multi-pass Ti:sapphire amplifiers for a continuous-wave single-mode laser", Optics Lett., vol. 16 (1991) pp. 144-146. However, the designs of these workers are fundamentally different in both implementation and effect from those of the subject invention. Khoroshilov et. al. and Georges et al use amplifier cells where a laser beam is multiply passed through a gain medium, and the beam is refocused between passes of the amplifier, as shown in FIG. 1. This geometry employs two mirrors that are ideally paraboloidal and of differing focal lengths deployed around the gain medium, with the mirrors positioned to have common foci and the gain medium being located at the common foci of the mirrors.
FIG. 1 illustrates a design disclosed by Georges et al, ibid. An optical amplifier cell 1.30 shown in FIG. 1 has a laser gain medium 101 of Ti:sapphire and has a central or resonator axis 104. A frequency doubled Nd:YAG laser beam 102 pumps the gain medium 101. The central axis 104 of the cell 130 passes through the gain medium 101. The path of a light beam passes through the gain medium 101 and is translated downward as a result of passing through two Brewster angle faces of the gain medium. A light beam 112 is introduced into the cell 130 parallel to, but offset by a distance d from, the central axis 104 of the cell, which axis is also the central axis of the pump bean 102. The input or probe beam 112 to be amplified passes through an aperture or hole in an inwardly facing paraboloidal or spherical mirror 105 of focal length F.sub.1. The beam 112 then passes outside the gain medium 101, where the beam is later amplified, and develops a beam waist in a transverse plane 107 that contains the mirror foci. This beam waist occurs because of beam tailoring optics external to the amplifier that are supplied by the user, a standard design problem for those skilled in optics.
The light beam 112 is incident on a paraboloidal or spherical mirror 106 of focal length F.sub.2, where F.sub.2 &lt;F.sub.1 so that the mirrors have unequal focal lengths. The beam 112 is reflected from the mirror 106 as a light beam 113 through the gain medium 101 to form another beam waist in the gain medium. An amplified light beam 114 passes out of the gain medium 101 and is incident upon and reflects from the mirror 105. The resulting reflected beam 115 propagates parallel to the central axis 104 of the cell, but offset from this axis by a distance of d(F.sub.1 /F.sub.2), which is greater than d. The beam 115 passes outside the gain 101 and develops a beam waist in the plane 107 of the beam foci. The beam 115 is then incident upon the mirror 106. The light beam 115 is reflected as a light beam 116 and proceeds toward the gain medium 101, where this beam is amplified as a light beam 117, and continues toward the mirror 105 and is reflected as a light beam 118. The light beam 118 approaches and is reflected from the mirror 106 as a light beam 119. The light beam 119 passes to another waist in the gain medium 101, where the beam is further amplified as a light beam 120. The beam 120 propagates parallel to the central axis 104 of the cell, but offset from it by a distance d.sub.offset of: EQU d.sub.offset =d(F.sub.1 /F.sub.2).sup.2. (1)
The beam is sequentially amplified and displaced from the central axis 104 in a geometric progression of displacements. After sufficient amplification the beam is extracted from the amplifier cell 130 by a mirror 108 to, produce a light beam output 109. If the mirror 108 is moved away from the central axis 104, the number of passes the beam makes within the amplifier cell is increased, if the mirrors 105 and 106 have sufficiently large diameters. The difference .DELTA..sub.n in the offset distance between adjacent beams on the same side of the central axis 104, separated by two consecutive beam passes through the gain medium 101, is EQU .DELTA..sub.n =d[(F.sub.1 /F.sub.2).sup.n+2 -(F.sub.1 /F.sub.2).sup.n ],(2)
where n is the number of passes of the light beam through the gain medium 101 for the earlier beam. Each time a light beam is displaced to a leg further removed from the central axis 103, the beam diameter at the waist in the collimated leg is also magnified by the ratio F.sub.1 /F.sub.2, and in the next pass of the beam through the gain medium the beam waist is demagnified by the inverse ratio: F.sub.2 /F.sub.1. In many situations this apparatus would produce superior results, if the beam diameter did not geometrically vary in this way, but rather was fixed at approximately the diameter of the pumped region of the gain medium 101. Mismatching the diameters of the light beam and the pumped region of the gain medium results in inefficient energy extraction and/or reduction of gain.
Optimally, the ratio F.sub.1 /F.sub.2 and the input beam waist radius are chosen so that the beam waists in the gain medium 101 on the later passes of the cell fill the pumped region of the cell in order to get useful energy extraction. Also the ratio F.sub.1 /F.sub.2 and the input beam waist radius need to be chosen so that the difference in the offset between adjacent beams on the same side of the central axis, given by Equation (2), is greater than about 2.5 beam diameters, to avoid clipping the beam and producing diffractive losses when the beam passes a sharp edge. In the geometry of FIG. 1, m optional mirror 110 and lens 111 are positioned to reflect unabsorbed pump light back through the gain medium 101, to maximize the absorption of the pump light. The fact that the light beam diameter increases geometrically after each pass through the amplifier cell 130 in the design of Georges et al shown in FIG. 1 makes it harder to achieve a large number of passes through the gain medium 101, because the beam radius rapidly becomes too large for the radii of the mirrors 105 and 106. The progression of beam diameters and locations, 121, 122, 123, 124 (not drawn to scale in FIG. 1) indicates how the beam diameters grow as the beam is translated outward from the central axis 104 of the amplifier cell 130.
LeBlanc et al, in "Compact and efficient multipass Ti:sapphire system . . . ", Optics Letters, vol. 18 (1993) pp. 140-142, discuss an eight-pass amplifier for femtosecond-length, chirped-pulse amplification in which the first four beam passes occur in a first plane and the second four beam passes occur in a parallel plane. This is a self-imaging system, but several mirrors are used for only a single reflection so that the system requires many mirrors and is quite complex.
Many techniques are available to amplify light, but such techniques are often restricted in their uses and cannot be used for several formats such as cw, Q-switched pulses, or mode-locked pulses. What is needed is a laser amplifier that is efficient, simple and cost-effective and offers high gain. Preferably, the apparatus should be able to amplify a beam with a Gaussian or near-Gaussian profile without profile degradation. Preferably, the amplifier geometry should have reduced sensitivity to thermal lensing and should accept a wide range of pumping mechanisms, including laser diode pumping.